Ammonium nitrate explosive containing hydrogen sulfide treated prussian blue



United States Patent M 3,137,598 AMMONIUM NITRATE EXPLOSIVE CONTAINING HBLYUEDROGEN SULFIDE TREATED PRUSSIAN Wayne A. Proell, Chicago, 111., and William G. Stanley, Hammond, Ind., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana N0 Drawing. Filed Dec. 12, 1952, Ser. No. 325,702 2 Claims. (Cl. 14946) This invention relates to new and improved compositions for the generation of gas at high pressure. Particularly, the invention relates to explosive compositions wherein ammonium nitrate is the principal or sole gasproducing agent. Still more particularly, the invention relates to an explosive comprising ammonium nitrate, an oxidizable material and a particular combustion catalyst.

Ammonium nitrate is widely used as a component of high explosives. Although ammonium nitrate is classified as a high explosive, it is extremely insensitive and cannot readily be detonated by the local application of heat or by a blasting cap; and when ignited, ammonium nitrate does not sustain propagation consistently. Normally ammonium nitrate is mixed with an oxidizable material, such as, sulfur, carbon, cellulosic materials,

hydrocarbons, etc., in order to utilize the excess oxygen available in the ammonium nitrate. However, these mixtures .of ammonium nitrate and oxidizable materials are also either very insensitive or slow burning.

One of the well known methods of overcoming this lack of sensitivity is to use a sensitive high explosive to prime the detonation of the ammonium nitrate explosive. Examples of suitable primers are tetryl, TNT, nitrostarch, nitrocellulose, nitroglycerine, etc. An explosive that is detonable by the action of a blasting cap can be obtained by mixing the ammonium nitrate with the sensitive materials, such as, nitrostarch and nitrocellulose. The extreme sensitivity of these explosives makes them undesirable for ordinary blasting use.

Another method of obtaining a readily ignitable ammonium nitrate explosive is to admix therewith an organic sensitizer, such as, nitrogen compounds and certain carbohydrates. In general, satisfactory explosives are obtained only when the sensitizer is very intimately dispersed throughout the mass. Generally this dispersion is obtainable only by the use of complicated and expensive procedures.

The most commonlyused method for improving the sensitivity of ammonium nitrate explosives is to add a combustion catalyst. The commercially used combustion catalysts are all based on the element chromium. The more common chromium combustion catalysts are ammonium or alkali metal chromates or polychromates; chromic oxide, chromic nitrate and copper chromite. The preferred material is ammonium dichromate. While the chromium compounds are the best known combustion catalysts, they have the disadvantages of being expensive and of, frequently, being in very short supply. The relative toxicity of the chromates makes them hazardous unless handled with considerable care.

It has been discovered that certain iron compounds at effective catalysts for the combustion of ammonium nitrate and ammonium nitrate-oxidizable material mixtures. These catalysts are the subject matter of US. patent applications filed by Wayne A. Proell and William G. Stanley, Serial Number 273,564, filed February 26, 1952, now Patent No. 2,987,389, and Serial Number 288,065, filed May 15, 1952, now Patent No. 2,955,033. All of the combustion catalysts disclosed in these applications contain the iron cyanide radical, either ferrocyanide or ferricyanide. In addition to the iron cyanide radical,

these catalysts contain a second iron ion which may be 3,137,598 Patented June 16, 1964 either ferric or ferro(us). In addition to the iron-iron cyanide complex, the catalyst may contain alkali metal and/ or ammonium ions. It has been found that the generic classes of iron-iron cyanide compounds known as soluble Prussian blues and insoluble Prussian blues are effective catalysts for the purposes of this invention. It is known that the better soluble Prussian blues contain alkali metal(s) such as potassium and sodium and/or the ammonium radical. Some of the compounds which have been found to be very effective are: ferric ferrocyanide, ferrous ferricyanide, potassium ferric ferrocyanide, sodium ferric ferrocyanide, ammonium ferric ferro- Icyanide, soluble Prussian blue, and insoluble Prussian The above applications disclose that insoluble Prussian blue is the more effective catalyst at elevated pressures such as exist in the combustion chamber of a rocket motor, e.g., 500 to 2000 p.s.i. However, the insoluble Prussian blue containing ammonium nitrate-oxidizable material mixture is very difiicult'to ignite at low pressure, e.g., about 15 p.s.i. when the catalyst content is about 6 weight percent or less. Also, considerable variation in catalytic activity exists in insoluble Prussian blue from different sources and event different batches.

An object of this invention is the preparation of a new and improved inorganic nitrate explosive, in particular, an ammonium nitrate explosive. Another object is the preparation of an explosive comprising ammonium nitrate and a combustion catalyst. A particular object of this invention is an explosive mixture comprising ammonium nitrate, an oxidizable material and a particular burning catalyst. A further object of this invention is a gas generating composition for the generation of gases at pressures on the order of 1000 p.s.i., which composition comprises ammonium nitrate, an oxidizable material and an H 5 treated-Prussian blue combustion catalyst.

The above objects and other objects which will be apparent in the detailed description of the invention are achieved as follows: The gas generating composition of this invention comprises a mixture of ammonium nitrate, or a mixture of ammonium nitrate and an oxidizable material, and an effective amount of a combustion catalyst consisting essentially of an H 5 treated-Prussian blue. In addition to the ammonium nitrate, moderate amounts of other inorganic nitrates, such as, potassium nitrate, sodium nitrate and magnesium nitrate may be present.

It is preferred to have present in the high pressure gas generating composition of this invention an oxidizable material, which material utilizes a part or all of the excess oxygen available from the decomposition of the ammonium nitrate and the combustion catalyst. (The catalyst uses some of the excess oxygen from the decomposition of the ammonium nitrate for the oxidation of the metal and carbon content thereof.)

The combustion catalyst of this invention is obtained by exposing iron-iron cyanide to the action of hydrogen sulfide gas. The H S is rapidly taken up for a time but the rate of sulfur addition decreases and finally stops. The point of completed sulfur addition is indicated fairly well by the fact that the temperature of the reaction zone reaches a constant level. The H 5 treated material has a strong odor of hydrogen sulfide if cooled to room temperature quickly after the interaction has come to an equilibrium, as indicated by a constant reaction zone temperature. The odor is lost slowly on prolonged standing. The amount of H 3 taken up will vary somewhat with the particular material charged. However, generally the catalyst will contain between about 2 and 7 weight percent of sulfur; preferably the catalyst will contain between about 4 and 5 weight percent of sulfur.

The term iron-iron cyanide material is intended to include ferric ferrocyanide and all the various grades of commercially available soluble and insoluble Prussian blue. The term H treated-insoluble Prussian blue is intended to include the product of the H streatment ofof ordinary insoluble Prussian blue as a combustion catalyst for ammonium nitrate. The H S treated-insoluble Prussian blue readily ignites at low pressures. Insoluble Prussian blue as obtained on the commercial market shows a considerable variation in catalytic activity from source to source and even from batch to batch from the 'same source; the H 8 treated-insoluble Prussian blue is of about the same catalytic activity regardless of the source of the insoluble Prussian blue used as the starting material.

When operating with ammonium nitrate in the absence 7 of any appreciable amount of oxidizable material, large amounts of catalyst are needed to permit smooth burning of the composition. For ignition at ordinary temperatures, the amount of catalyst needed may be as much as 10 weight percent of the total explosive mixture. 'However, the ease of ignitability of the mixture increases with increase of pressure in the gas generating chamber. At

elevated pressures, ignition occurs readily even when i using small amounts of combustion catalyst. The term ordinary pressure is intended to mean pressure such as exists normally in the atmosphere or in mining operations.

When the gas generating composition consists essentially of ammonium nitrate, an oxidizable material and the combustion catalyst of this invention, the combustion catalyst may be present in an amount between about 1 and 25 weight percent, based on the total'composition. In order to improve the ignitability of the composition and obtain smoother burning thereof, it is preferred to use at least about 2% of catalyst. The burning rate is somewhat affected by the amount of catalyst present in the composition; however, the maximum effective concentration of the catalyst is about 15%. The amount oficatalyst needed to obtain good ignitability and smooth operation is dependent somewhat on the amount and type of oxidizable material present; in general between about 2 and 4 weight percent will give excellent results when maximum burning rate' is not a main consideration. When maximum burning rate is a main consideration, the amount of catalyst used should be between about 6 and 15 weight percent.

The decomposition of ammonium nitrate produces free oxygen in addition to the other decomposition products.

Additional energy can be obtained by having present in a the combustion zone an oxidizable material which combines with the free oxygen. It is to be understood that" the combustion catalysts utilized in the explosive grain of this invention contain oxidizable materials which will consume some of the free oxygen. The oxidizable material may be any material which contains a deficiency of combined oxygen. nesium may be used. Thenon-metallic elements, sulfur and carbon, may beused. Nitrogen-containing organic compounds that do not unduly sensitize the explosive mixture are particularly good; examples of these are urea, nitroguanidine, mononitro naphthalene, dinitrodiphenyl oxide, etc. Hydrocarbon materials are an excellent oxidizable material and are preferred where low cost is a matter of concern. Examples of hydrocarbonoxidiza- Metals such as aluminum and mag- It has been found that the various cellulose preponderate nitrate.

The explosive mixture can be made by milling the in-' v 4 V is desirable to .use an. oxygenated plasticizer and a polyester of a glycol and a dicarboxylic acid is a particularly suitable plasticizer material. The term oxygenated oxidizable materia is intended to include all organic ma;

terials which contain oxygen as. well as carbon and hy-l drogen-other elements such as nitrogen and sulfur may a also be present.

The amount of oxidizable material that'is added to the ammonium nitratefcatalyst mixture is dependent upon the amount of catalyst present and the particular type of:

oxidizable material itself.- Normally it is desirable to have present a suflicientamount of oxidizable material so' that the explosive mixture is about stoichiometrically balanced with respect to oxygen content. This preferred composition may be exceeded, but in general no'more oxidizable material should be present than can react with the available oxygen to yield a soot-free gas. The presence of some oxidizable material is desirable so that the explosive mixture of this invention should contain'be-j tween about 5 and 25% of oxidizable material, preferably between about 15 and 25% of oxidizable material. 7,

The term ammonium nitrate as'used in this specification and in the claims'is-intendedto mean either ordinary commercial grade ammonium nitrate,.such as, conventionally grained ammonium-nitrate containing a small amount of impurities and which is then generally coated with a small amount of moisture-resisting material such as petrolatum or paraffin, or military grade ammonium nitrate, or a mixture of other inorganic nitrates and ammonium nitrate wherein the ammonium nitrate is .the

gredients or by dry mixing; this op'eration is preferably followed by forming regular shaped compacted grainsby pressing the powdered mix in molds, Itis preferred to prepare the shaped grains by adding th powdered am- 7 monium nitrate to fused organic or oxidizable materialat 100-125 C., mixing to form a paste, and pressing the paste intosuitable molds. The cooled grains are strong i at that point.

and durable.

g The efiectiveness of H 8 treated-Prussian blue as acombustion catalyst is-illustrated belowJ Severalgrams r of an insoluble Prussian blue (essentially'pure ferricferrocyanide) having a sulfur content of 0 .16 Weight percent were placed into a flask. The flask and contents were at room temperature, about 20 C. Free H 8 gas was slowly passed through the finely powdered mass of insoluble Prussian blue. The temperature of thereaction zone-as measured by a thermometer inserted into the powder immediately. increased and continued to increase fairly rapidly to about a temperature of about C. Thetemperature increase slowed down at this point and slowly rose to about C. and became substantially constant its contents were cooled toroom temperature. .The blue color of the insoluble Prussian blue had changed 'to'a dark green color- The powder was analyzed and was found to contain 4.26 weight percent of sulfur. TheHgS treatedinsoluble Prussian blue'had a very strong odor of H 8 at room temperature even after many hours standing in 7 an open beaker. i i

A satisfactory catalyst can be made by slurrying the iron-iron cyanide in water or hydrocarbon and passing the hydrogen sulfide through'the slurry until either the desired sulfur content has been reached or the sulfur addicontents thereof were thoroughly mixed. The mixture 7 was extruded by means of a laboratory-size extruder to form a grain about 5/ inch in diameter and Gincheslong.

g D The H 8 gas streamwas turned on when the reaction zone temperature leveledout. .The flask and In order to insure uniformity, duplicate grains were made and tested in each example. The burning characteristics of each explosive mixture were determined after inhibiting the cylindrical surface of the grain with a thin layer of either asphalt or a cellulose base thermoplastic such as cellulose methacrylate. Burning rates were determined at atmospheric pressure by igniting the end of the grain and placing a beaker over the ignited grain, thus simulating burning in an inert atmosphere. For tests at elevated pressures, the grain was placed in a Crawford bomb and the bomb brought to the desired operating pressure by means of cylinder nitrogen prior to igniting the grain. Each burning rate represents an average of at least two trials.

T est 1 In this test the grains consisted of, on a weight basis, ammonium nitrate, 73.5%; cellulose acetate, 6.8% dinitrodiphenyl oxide, 6.8%; dimethyl phthalate, 6.8%; and catalyst, 6.0%. Grain A contained the H 8 treatedinsoluble Prussian blue prepared as described above. Grain B contained insoluble Prussian blue from the same batch that had formed the charge to the preparation of the H S treated-insoluble Prussian blue. At atmospheric pressure grain A ignited readily and gave a burning rate it of 0.009 inch per second. At atmospheric pressure Grain B could not be ignited.

Test 2 In this test the grains consisted of, on a Weight percent basis, ammonium nitrate, 80.5%; a F. softening point asphalt, 15.5% and catalyst, 4.0%. Grain C used as a catalyst a commercial grade soluble Prussian blue which had been treated with H S gas as described above. At atmospheric pressure grain C ignited readily and burned at a rate of 0.006 inch per second. Grain D which contained untreated soluble Prussian blue could not be ignited.

Thus having described the invention, what is claimed is:

1. An explosive composition which comprises between about 5 and 25 weight percent of an oxidizable material, between about 1 and 25 weight percent of an H S treated insoluble Prussian blue characterized by a sulfur content between about 2 and 7 weight percent, and the remainder essentially ammonium nitrate.

2. The composition of claim 1 wherein said catalyst contains between about 4 and 5 weight percent of sulfur.

References Cited in the file of this patent UNITED STATES PATENTS 2,879,133 Marti Mar. 24, 1959 

1. AN EXPLOSIVE COMPOSITION WHICH COMPRISES BETWEEN ABOUT 5 AND 25 WEIGHT PERCENT OF AN OXIDAZIABLE MATERIAL, BETWEEN ABOUT 1 AND 25 WEIGHT PERCENT OF AN H2S TREATED INSOLUBLE PRUSSIAN BLUE CHARACTERIZED BY A SULFUR CONTENT BETWEEN ABOUT 2 AND 7 WEIGHT PERCENT, AND THE REMAINDER ESSENTIALLY AMMONIUM NITRATE. 